Seatbelt-mounted airbag is a new type of occupant restraint system, in which the airbag is integrated into the seatbelt and hence can be easily and quickly implemented into the current tactical vehicles without significant vehicle structure or interior changes. The objective of this study was to develop, optimize, and demonstrate seatbelt-mounted airbag designs for reducing occupant injury risks in a light tactical vehicle under frontal crashes. A total of 19 sled tests and over 30 FE simulations were performed to find the optimal seatbelt-mounted airbag designs for protecting occupants represented by three sizes of ATDs and two military gear configurations. Various lap-belt-mounted airbag and shoulder-belt-mounted airbag designs were evaluated for driver, front-seat passenger, and rear-seat passenger locations in a tactical vehicle. The test and simulation results showed that the optimized designs substantially reduced the occupant injury risks to the head, neck, and chest compared to the baseline tests. This study demonstrated the benefit of adding a properly designed seatbelt-mounted airbag to improve the occupant protection in frontal crashes under an environment representing a light tactical vehicle.